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Why do wings prefer laminar flow if cd is lower for turbulent flows?
Can anyone please explain to me why we prefer to maintain laminar flow over wings, despite the fact that the overall coefficient of drag appears to reduce as the Reynolds Number increases? I have read that turbulent flows lead to increased skin friction drag, but this is hard to internalize given that the cd is lower overall in turbulent flows for the same angle of attack.
I've been trying to do some research to better understand why aircraft are designed this way, but I haven't had much luck so far. If anyone can offer me some insight here, it would be greatly appreciated!
As an example, I thought it would help to include the drag polar for the NACA 0012, obtained from airfoiltools (http://airfoiltools.com/airfoil/details?airfoil=n0012-il).

Laminar flow and turbulent flow are not directly linked to Reynolds number, even though Reynolds number is a heuristic indicator of when the flow may transition from laminar to turbulent. However, the transition is actually not well-understood and is difficult to model/predict. That's why you will see cases where, for the same Reynolds number, one flow is laminar and another is turbulent.
Let's take a flat plate for an example. The flat plate boundary layer in laminar flow actually has a closed-form solution; the skin friction drag is given by:
$$C_{f}=\frac{1.328}{Re_c^{1/2}}$$
As you can see, as Reynolds number increases, the drag coefficient decreases.
Turbulent flow, on the other hand, has no closed-form solution. An approximation for a smooth plate is given by:
$$C_{f}=\frac{0.074}{Re_c^{1/5}}$$
The two lines are plotted in the figure below (cited from this Penn State course, which has a pretty good summary of boundary layer results for flat plate). As you can see, at the same Re, laminar flow has less skin friction drag than the turbulent counterpart.

From a skin friction drag perspective, if you can keep the wing in laminar flow, then all the power to you. In practice, however, this is very difficult. Any un-eveness on the wing (e.g. rivets, steps and gaps) could transition it to turbulent flow downstream. Any contamination, such as insects, could potentially ruin it.
Another consideration is whether staying laminar for the whole wing is desirable from a flow separation perspective, since laminar flow separates easier than turbulent flow. There will be a trade-off amongst stall speed, high AOA handling and drag.

Ah - I see now. This helps immensely! Thank you so much :)

"There will be a trade-off amongst stall speed, high AOA handling and drag" - and high-mach behaviour.

Can anyone please explain to me why we prefer to maintain laminar flow over wings, despite the fact that the overall coefficient of drag appears to reduce as the Reynolds Number increases?

The drag coefficient goes indeed down with increasing Reynolds number for both boundary layer conditions, turbulent and laminar flow. Generally, a laminar boundary layer is preferable as long as the flow is accelerated, because viscous drag will be lower, and a turbulent one is better when it is decelerated, so separation can be delayed.

turbulent flows lead to increased skin friction drag

That is correct. In case you wonder why: Shear stress, which is causing friction drag, is proportional to the speed gradient at the wall. The sketch below illustrates the difference (picture source) by plotting flow speed over height of the surface:

This means:

A thin boundary layer produces more shear than a thick one. The boundary layer is thinnest right next to the stagnation point and grows in thickness downstream.
A turbulent boundary layer will create much more shear stress than a laminar one. Most of the shear on an airfoil happens past the transition point.
Higher speeds cause higher shear stresses. Therefore, the suction area on the upper side of the airfoil produces more shear stress than the pressure area on the lower side. In a separation bubble with speed reversal at the wall you will even get a small amount of "shear thrust".

Friction drag over chord for an E502mod airfoil at 3° AoA (own work). Blue: Top surface, Red: Bottom surface. Source: XFOIL 6.97. The transition points are clearly visible: It is where shear shoots up again at mid-chord. Note the separation bubble ahead of the transition point on the bottom side: It is marked by negative values of shear.

Why do wings prefer laminar flow if cd is lower for turbulent flows?

As shown above, it is the opposite: Laminar flow produces less drag than turbulent flow. Exceptions apply.


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How can an helicopter end up with the same tail number than an airplane?
For example, take N133AA.
https://www.jetphotos.com/registration/N133AA shows American Airlines photos from the 90s and an helicopter from this decade:

The FAA only shows the helicopter registry:

https://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=N133AA

Transtats shows that the last time N133AA flew from LA to NY was in 2008, and I'd bet that was not the helicopter.
Is it normal that tail numbers get reused? How can I make my cross-searches reliable?

It's very normal for aircraft registrations to be reused, in many countries. See some examples https://www.airliners.net/forum/viewtopic.php?t=742769

N133AA appeared on an American Airlines DC10, but American Airlines retired, sold or scrapped all their DC10 and MD11 aircraft by 2002, releasing the registrations for other uses.
Wind the clock forward 15 years and somebody has used N133AA for a Robinson R44 helicopter. I speculate that the effective owner of the helicopter is one Agustin Anaya, who submitted the photographs to JetPhotos, even though the aircraft is registered to a company.
The FAA registry shows only the helicopter because the American Airlines usage was released long ago.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


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Can accelerations be used to calculate load factors?
Can I use the z accelerations to calculate the load factor in that axis? Please explain how or why not.

Highly related to this other recent question/answer: https://aviation.stackexchange.com/questions/71511/how-does-the-load-factor-vary-when-the-aircraft-pitches-up-do

It sure can; in fact, they are equivalent (with an offset of 1G).
In the body frame, the vertical equation of motion is:
$$Z+mg\cos\theta\cos\phi=m(\dot{w}+pv-qu)$$
Accelerometers can't measure inertial or gravitational forces, so vertical acceleration as measured is $N_z=Z/m$.

What do you mean by "accelerometers can't measure inertial or gravitational forces"? That's exactly what they measure.

@Zeus I mean exactly what I wrote. An accelerometer under free fall will measure no force.

Yes, and there will be no load. This is precisely because it measures both the gravitational force and the inertial force of the acceleration, which add up to zero. A static accelerometer will measure the gravitational force (acting on its sensor) at its location and will not show zero, despite zero acceleration.

@Zeus A static accelerometer on a table top shows 1G because it's measuring the normal force (Z in the expression). If it's measuring the gravity component as well, then it should read zero. What I wrote and what you wrote above are equivalent.

Well, you can say it's a philosophical question, because you can always claim that the accelerometer 'actually' measures the reaction of its internal spring (if we assume a classic weight accelerometer). But this is an 'implementation detail'; it's meant to measure the gravitational (and inertial) force.

@Zeus This is not merely an implementation detail. Inertial forces (e.g. coriolis, centripetal) aren't actually forces; that's why they are called fictitious forces. Gravitational forces are special too, hence the elevator equivalence principle.

They are not 'actually' forces, but an accelerometers measures exactly them, rather than the acceleration per se. In an orbit/turn, it will indicate a force away from the centre (centrifugal), instead of the centripetal acceleration that causes it. It is a matter of our interpretation to move from one to the other.

@Zeus I don't want to dwell on this since it's offering no value to the question. Obviously, the inertial forces cannot exist without the external forces. But the point is, you "measure" one or the other. I choose to express measurement as body forces minus gravity here. Moved to chat: https://chat.stackexchange.com/rooms/100915/discussion-between-jimmy-and-zeus

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Formula for calculating turn indicator (deviation from coordination)
being more a professional programmer and less prof pilot, I am building a small
Android EFIS and I'm able to calculate quaternions, yaw, pitch, roll, TAS, acceleration values etc. I am stuck calculating the deviation from a coordinated curve indicator, usually the small ball in the turn indicator. Could someone please point me to a formula? I'm finding only formulas about calculating the necessary bank angle or radius from speed.
Thanks in advance and best regards
hk

A filtered lateral acceleration would suffice. Turn indicator does not indicate sideslip anyway.

I am stuck calculating the deviation from a coordinated curve indicator, usually the small ball in the turn indicator.

The "small ball in the turn indicator" (which pilots use to determine what they need to do with the rudder pedals in order to stay in coordinated flight) is just an inclinometer. It's literally just a heavy ball in a curved tube filled with oil.
If you want to replicate the way that the inclinometer works, then look at the lateral acceleration and vertical acceleration and use the arctan2 function to determine the apparent tilt.
By "lateral acceleration," I mean the proper acceleration in the direction of the aircraft's lateral axis, since that's what matters here and that's what you're able to sense. Likewise, by "vertical acceleration," I mean proper acceleration in the direction of the vertical axis.
The oil in the tube functions as a low-pass filter with a time constant of maybe about a second.
All that said, I don't think there's actually any reason to use the vertical acceleration. You can just take the lateral acceleration, low-pass filter it, and display the result.

Thank's guys, but 1 more question. In what coordination system the lateral acceleration is defined? In the banked aircraft's or earth' ?

@hans In the banked aircraft's coordinate system. I've edited to clarify that.


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Are a gyrodyne and a compound helicopter the same thing?
Both the gyrodyne and the compound helicopter are typically rotorcraft with driven rotor, additional stub wings and thrust provision. Are they two terms for the same thing or do they differ in some way?

Five seconds of Google search and answer is no. A gyrodyne works like a helicopter for lift off and landing and like an autogyro for flight.

And no Google or Wikipedia is not always reliable but in this case the derivation of the name strongly suggest that unlike a compound helicopter a gyrodyne gets force from the autogyro effect as an intrinsic part of its operation.

So there you go. Google says no, Wikipedia says yes, neither is reliable. Not a bad reason for coming here to ask, really.

I have now found a couple of pretty authoritative definitions for a gyroplane:

FAA Rotorcraft Flying handbook: For Gyroplane Use Only:

"The 'gyrodyne' is a hybrid rotorcraft that is capable of hovering and yet cruises in autorotation. The first successful example of this type of aircraft was the British Fairy Rotodyne, certificated
  to the Transport Category in 1958. During the 1960s
  and 1970s, the popularity of gyroplanes increased with
  the certification of the McCulloch J-2 and Umbaugh.
  The latter becoming the Air & Space 18A."

Electronic Code of Federal Regulations (e-CFR) Title 14. Aeronautics and Space Chapter I. FEDERAL AVIATION ADMINISTRATION, DEPARTMENT OF TRANSPORTATION Subchapter A. DEFINITIONS AND GENERAL REQUIREMENTS Part 1. DEFINITIONS AND ABBREVIATIONS, Section 1.1. General Legal Information Institute, Cornell Law School.

"Gyrodyne means a rotorcraft whose rotors are normally engine-driven for takeoff, hovering, and landing, and for forward flight through part of its speed range, and whose means of propulsion, consisting usually of conventional propellers, is independent of the rotor system."

So it may be that the gyrodyne fully offloads the rotor and relies on autorotation during cruise, while the compound helicopter drives its rotor throughout. But I can find no authoritative definition of a compound helicopter.
Technically, under autorotation the rotor disc tilts backwards and air flows up and over the blades from below, while under powered drive the rotor disc tilts forwards and the blades drive air downwards from above. It would be nice to think that this was reflected in the two definitions, but there are obviously borderline cases where the disc angle and loading are not quite textbook.

A gyrodyne is a type of VTOL aircraft with a helicopter rotor-like system that is driven by its engine for takeoff and landing and also includes one or more conventional propellers to provide forward thrust during cruising flight. Lift during forward flight is provided by a combination of the rotor, like an autogyro, and conventional wings. Due to a number of issues, there is some confusion over the term "gyrodyne", and the terms compound helicopter and compound gyroplane are frequently used to describe the same design. The gyrodyne is one of a number of similar concepts which attempt to provide helicopter-like low-speed performance and conventional fixed-wing high-speeds

Source :https://en.m.wikipedia.org/wiki/Gyrodyne
Because wikipédia is not a reliable source this site supports his arguments: https://www.helis.com/howflies/compound.php

Would that one could always rely on Wikipedia. But even Wikipedia does not regard itself as a reliable source.

You are right but a lot of site supports his arguments in this situation


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How is wing area defined?
When the plain "wing area" is given in basic aircraft specifications, what area does it usually refer to? Is the same area used for calculating the wing loading?
For example I have a fuller specification in front of me which gives areas for:

Main plane with ailerons
Main plane with ailerons and body centre section
Aileron, total
Tail plane
Elevators, two

If this were being condensed to a single value for "wing area", which of these would be used? Obviously, for something with a very long root chord such as a delta-wing jet, the areas with and without centre section will be very different, also leading to very different calculated wing loadings.
Has standard practice changed over the years?

Yes, the plain wing area is also used for wing loading. How the value is calculaed depends on the method used. In all cases the center section and ailerons are included but the horizontal tail is excluded. Common is also the use of the projected area of the clean wing in the x-y-plane, because that is the area relevant for lift.
For airliners, the two most important methods are the Wimpress and the Airbus method (the Wimpress being used by Boeing). The difference is the triangles you get by extending the wing's leading and trailing edges forward into the fuselage for Wimpress versus connecting the points where the leading and trailing edges meet the fuselage on both sides by two straight lines for Airbus. This discussion on airliners.net lists a comparison of the wing areas you get with both methods for a range of models.

Wing area methods in comparison. The shaded are is covered by the fuselage and determined differently; creating different areas for the same wing.
Note that the specifics of the method used is irrelevant for the purpose: This wing area serves as the reference area of most aerodynamic coefficients and either method is good enough for the purpose. Consistency is more important than precision here.
Both methods have been around for a while, so there was no change recently. But since both are used in parallel, there is no common standard.

The C of A for DH.88 Comet Racer G-ACSS of 1934 requires the "Area of main planes" and this has been filled in with the area of the wings only, lacking the centre section, so different again. But was that technically the same meaning as "wing area" in those days?

@GuyInchbald: When even today there is no common standard, i would expect that back then an individual engineer would use different ways to compute wing area, depending what he was working on. One that excludes the center section looks entirely plausible.

So, when we calculate the lifting force, should we include the shaded area as the wing's part (included as wing area) regardless of the different between Airbus and Boeing's method?

@AirCraftLover Yes, the shaded area needs to be included. You need to correct for the Mitteneffekt and the lift loss due to the presence of the fuselage anyway, so any method is fine when the proper corrections are applied, too.

Thank you for your confirmation and that additional info.


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Why does exhaust flow inwards through a radial turbocharger?
In a radial compressor, the air enters centrally and passes outwards, aided by centrifugal force. But, as the diagram shows, in piston-engine superchargers the driving exhaust gas flows inwards through a radial turbine. This has to fight against centrifugal force instead of leveraging it. So why is it done?

This has to fight against centrifugal force instead of leveraging it.

In fact, the opposite is true. The arrangement shown actually helps to extract energy, perhaps counterintuitively. A typical example of this arrangement is the Francis turbine.
A high flow speed plus high tangential velocity input, fed through to a low flow speed/low tangential velocity exit, will create the maximum energy differential for extraction. And that describes inwards flow.
There are a few ways of looking at this. One is to look at the exit velocity of the fluid. In a compressor, it's okay if the fluid has some leftover velocity as it will by converted to static pressure when slowed down by Bernoulli's law. For a turbine, the exit velocity should be as low as possible, as kinetic energy of the exhaust flow is 'wasted' energy. So it makes sense to place the exit at the inner radius of the turbine, where the radial velocity component is lowest.
Another way of looking at it is pressure. Centrifugal force creates a pressure gradient across the turbine in radial direction. As a 'packet' of air moves inwards, its pressure becomes lower. By conservation of energy, this must mean that it is expending work on its surroundings. The turbine extracts this work done from the packet of air moving inside.
Another intuitive explanation involves an ice skater retracting their arms in a pirouette (or a bored office worker on a swivelling chair). By conservation of angular momentum, their angular velocity increases. A packet of air would 'like' to increase its angular velocity, but this is kept fixed by the turbine rotating at a fixed velocity. The packet of air must thus be slowed down moving inwards, and as a result exerts a pressure on the turbine blade in front of it.
The choice between axial, centrifugal or mixed-flow is done based on the flow conditions, with high-flow low pressure differential favouring axial design and lower flow or higher pressure differential favouring the radial setup as shown. A more compact design means fewer turbine or compressor stages, which results in a higher pressure differential across the single stage. Thus, a radial setup is preferred.

Thank you. I have some observations. If the exit velocity of a compressor can be manipulated by design, so can that of a turbine. Then, whichever way the air flows its angular momentum will change, it doesn't matter which direction it pushes on the blades. So those points are not significant. But as you say, "The turbine extracts [the] work done from the packet of air moving inside." A high flow speed plus high tangential velocity input, fed through to a low flow speed/low tangential velocity exit, will create the maximum energy differential for extraction. And that describes inwards flow.

Exactly. Indeed you could swirl the turbine blades such that the flow is slowed down towards the outer perimeter but then you are limited by practical engineering (a larger blade so more wetted=draggy area). The reverse is just easier.

It's not obvious why the radial flow speed is lowest at the axis. In fact, due to the constricting effect (less volume near the axis), by Bernoulli we'd expect a higher radial speed.

@MSalters For most rotating devices, it is obvious that radial speed is greatest at the perimeter, lowest at the axle. You are right that, for turbomachinery, you could indeed find that the channels are directed forward or aft, effectively changing the radial speed. Your intuition is precisely the reason why this turbine is so effective: we would expect by Bernoulli (which is really conservation of energy) that the radial speed is higher, but instead the turbine slows down the radial speed and thus extracts work. See also my last comment.

@Sanchises: I still find it non-obvious. The radial velocity at a distance R by definition needs to be the volume flow divided by the cross-section at distance R. That cross-section, for a radial flow in a cylinder of length L, has an area  2πRL. For a constant volume flow, that means the radial velocity is proportional to 1/R. I.e. the velocity goes up. Of course, this is under the assumption of a constant volume flow. In reality, the mass flow is constant, and the pressure drops, so the volume flow only increases going inward. Hence the increase in radial velocity has to exceed 1/R.

@MSalters The reason the pressure drops is because the cross-sectional area of the flow is actually increasing. The air enters through a narrow slot in the inner wall of the feed duct and exits through a large-diameter exhaust duct. In this way the volume relationship you assume is defeated by design.

@MSalters It's indeed only 'obvious' if you don't think too hard about it :) I worked from the assumption of constant channel cross-section. While this may look difficult to achieve in practice, remember that the cross-sectional area of the flow is defined perpendicular to the flow. By curving the channels (which is a natural thing to do since the flow is not flowing radially initially) you can easily control the cross-sectional area to suit the flow.

It does seem counterintuitive to arrange the flow as it has been done in turbochargers, but this arrangement is the best compromise for harnessing kinetic energy from exhaust gasses of a reciprocating engine.
Design goals are

compact size
small backpressure
simplicity of construction (build and maintenance cost)
ability to utilize "traditional" materials and manufacturing tehcniques.

The turbine housing is formed in such a way, that it dimishimes in size as it circles the turbine wheel. Thus the exhaus gasses travelling throug the chamber eventually (rather quickly in fact) run out of space, and are pushed into the exhaust outlet by the following exhaust pulses.
As such, the exhaust gasses do not need to overcome or fight centrifugal forces, as the housing provides a path that guides the gasses along.
This arrangement, as opposed to reversing the flow, allows for a simplier and less critical turbine wheel profile, and much higher efficiency across flow rate range (iirc I try to dig up the reference) Compared to an axial turbine this construction is less efficient, but it also is so much more simple and compact, that it by far outweights the smaller efficiency.

Thank you. A couple of problems I have with that. 1) Does not the circling air path still create a centrifugal force and hence an unwelcome backpressure component? 2) If this is all so great for the turbine, why is it not equally great for the compressor?

One way to look at it is that the logic is similar for a regular fan. If you want the fan to blow air the curved (concave) part of the fan should be behind the fan. If you want to blow air into the fan the curved part of the fan should be in front of the fan. Your question is analogous to asking if it's so great for the fan to blow in the direction of the curve why is it less efficient for a windmill to receive wind in the same direction?


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Cleared straight in approach
I was flying IFR (in a simulator with Vatsim ATC) cleared direct to ROXXE on the RNAV 04 at KAVP at 4000 on a course of 275. The controller said "Cross ROXXE at or above 4000, cleared straight in approach RNAV 04". On a course of 275 I expected to enter the procedure turn at ROXXE but instead I was given a clearance that involved a 129 degree turn to the right to line up on final. Would this be a real world clearance in this situation or was the controller shortcutting? If real, how would I be expected to fly this?

This is an incorrect clearance. Per FAA Order 7110.65AA 4-8-1, the intercept angle must be 90 degrees or less for ATC to clear you for a straight-in approach:

d. Intercept angles greater than 90 degrees may be used when a procedure turn, a hold-in-lieu of procedure
turn pattern, or arrival holding is depicted and the pilot will execute the procedure.
e. If a procedure turn, hold-in-lieu of procedure turn, or arrival holding pattern is depicted and the angle of
intercept is 90 degrees or less, the aircraft must be instructed to conduct a straight-in approach if ATC does not
want the pilot to execute a procedure turn or hold-in-lieu of procedure turn. (See FIG 4−8−3)

ATC can make mistakes. If they clear you do something you don't think is safe and legal, reject the clearance and request an amended one. "Unable straight-in, request full approach" or similar would get the job done.


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What exactly is "plunge?"
I'm doing a project in which i'm attempting to model the motion of a wing using a system of second order differential equations. This is a 2-degree-of-freedom model based on the quantities pitch, a(t), and plunge, h(t). I know what pitch is, but what is plunge? and why is it affected by pitch? Where do these equations come from?

This looks like a flutter system of a representative wing section. Plunge is the vertical degree of freedom as shown in the diagram below (the other degree of freedom is pitch, $\theta$ or $\alpha$ in your assignment). You also have structural damping terms, which are omitted in the diagram.

Image ref: https://www.researchgate.net/figure/Sketch-of-airfoil-aeroelastic-system_fig1_313820106


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where can I find the codes used on Skyvector charts?
on https://skyvector.com/?ll=51.25181634927654,3.6845398064171233&chart=301&zoom=1 at N51°14.00' E3°46.96' is see a code "EBW" (blue circle with in-pointing teeth). I searched everywhere but couldn't find what "EBW" stands for. On Google maps I only see agricultural land there.
I also see similar codes like EHM and EBM, and would like to find an index for such codes.

The EBR and EBM codes stand for Europe Belgium Restricted / Military airspace (see AIP 5.1 and AIP 5.2). Likewise EHM is the same for the Netherlands (all Dutch ICAO codes start with EH). I guess EBW would refer to a warning then, but I'm not sure since I couldn't find anything in the AIP.


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Why aren't 757 scimitar winglets flush with the ailerons?
Over my many past flights aboard 757s, I have seen changes being implemented with their design. One of which was notably their refit of scimitar winglets. Although one of their most notable characteristics is the extended pointy edge on their tip, I have noticed that the inner edge of the winglet (past the wing position light) does not run flush with the aileron, unlike that of the older 757 winglets.
What is the reason behind this design exactly?
Thanks!
*Please note that the first 3 photos below are of the scimitar winglet and the last is of the original older design
Photo credits: First 3 photos are taken by me personally. Last one is from: https://www.flickr.com/photos/33415137@N06/6151948806

If I'm not mistaken, the newer winglet looks a little wider (chord).  Either that or the horizontal wing chord was reduced (more unlikely).  Looks like a redesigned winglet.

Having the winglet slightly "off" the wing tip may help reduce drag.

No prior knowledge of this design choice, but the design direction seems pretty clear here.
The original design reduces load towards the wingtip for efficiency. The textbook spanwise loading for maximum efficiency is the elliptical loading, however when tip devices, as in the original design, are employed, the tip load should not be zero. With a tip device, it is possible to maintain a pressure surface <> suction surface pressure differential. Therefore at the wing tip (just before the tip device), there is still significant lift being generated.
The scimitar winglet design that you have shown must be more efficient than the original design (otherwise why else would they retrofit it?), which therefore means that it can isolate a greater pressure differential from the outboard pressure surface to the outboard suction surface. Because this tip device is designed for a greater pressure <> suction surface pressure differential, but the original design was designed for a lesser differential, more load can now be generated outboard for the same drag penalty. As can be seen in the picture, the chord extension is at an incline to the trailing edge. This gives more camber to the outboard section of the wing, and therefore more lift on this portion and nearby.
Of course, the designer could have just tilted the trailing edge down for the same chord length as the inboard section of the wing. This would create the increase in outboard wing loading, but due to the abrupt change in camber from the aileron to the cambered outboard section, there would be a significant vortex formed. To decrease the effect of the vortex formation (it will still generate, but will be weaker), the surface follows the original design, but is extended with significantly more camber after the original wings trailing edge. The trailing edge camber (extra downwards curve) is very apparent in the first and second images. This device therefore allows an increase in outboard load without having to affect the inboard section geometry.
Obviously just creating more lift outboard, and therefore more overall lift, doesn't necessarily help an aircraft. During cruise, we only need sufficient lift to overcome the weight force. Because the new tip device creates more lift for the same amount of drag at a particular incidence, and because the drag polar is relatively unaffected, this means that we can create the same lift at a slightly different incidence angle, and therefore can cruise at a lower drag penalty.


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Do the blades of the rear disk have different geometry to the blades of the front disk on a contra-rotating assembly?
If my understanding is correct, the air flow is accelerated by the front propeller toward the rear propeller in a contra-rotating prop assembly. Therefore, the rear disk would be receiving air at different pressure and velocity than the front disk. Does this have an effect on the efficiency of the rear disk, and if so, do the rear blades have different geometry to the front blades?

I think the last question, do the rear blades have different geometry to the front blades?, is a false dichotomy. Some designs could be different, some designs are not.

The conditions at the rear disk are obviously different, so something must be different.
First of all, if the rotation direction of the disks is opposite, you'll have 'right' and 'left' blades. (Blades nearly always have asymmetric airfoil, so you can't swap them). That's 'different geometry' already.
Normally, the efficiency of the rear disk will be somewhat lower (zymhan's answer provides some sources). If so, an optimal design should provide more power/torque to the front disk. The most powerful turboprop NK-12 delivers about 20% more torque to the front disk.

Source
This doesn't mean, however, that the blades must be substantially different (other than mirrored). Indeed, on most coaxial assemblies they are the same (including the aforementioned NK-12). How does it work? All these props are constant speed units (at least, I'm not aware of others). The governors will simply set the appropriate blade pitch such that the RPMs were the same, despite different torque and conditions. This means in practice that the blade pitch on the rear disk will be lower. This may count as 'different geometry'.
Finally, there are rare examples when the disks are substantially different. One of them is the SV-27 propfan on Antonov-70, which has even different number of blades (8 vs 6). (This, I believe, is a prop with the highest disk loading ever).

Wiki source

propellers in the mist …

From NASA Technical Memorandum 82983

Swirl recovery reduces the magnitude of efficiency degradations at high
  power loadings and/or low tip speeds typically associated with
  single rotation propellers. Therefore,the ideal efficiency advantage
  offered by counter rotation propellers varies from about 5 to 15 units
  of efficiency for the range of conditions shown in figure 4.

Figure 10 also shows the relationship of blades per disk and tip sweep angle on efficiency

But this doesn't answer the main question stated in the title: does it follow that the rear prop actually has a different geometry?

@Zeus it is not one or the other. Some rear propeller air foils could be different from the front, while others may not be. There is no rule, or evidence, that the geometries are only different OR the same.


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Will a quad fan arrangement on an aircraft help with pitching moments during VTOL flight transition?
The phenomena of ducted fan assemblies inducing pitching moments whilst in transition from vertical to horizontal flight paths is a problem I have seen mentioned by many designers and engineers of VTOL aircraft. I am wondering if the quadratic arrangement(a fan placed at each of the outermost corners of the aircraft) would help alleviate these issues. It would seem to make sense as the front and rear fan pitching moments about a central CG would cancel each other out, though still causing stresses on the airframe.
Are these pitching moments still a problem in quad fan configurations, and has any research been done into the inlets of ducted fans to stop these pitching moments?

The Doak VZ-4 twin ducted propulsor flew in 1958, the Bell X-22 took that up to four in 1966. So yes, people have been thinking about it for over half a century.
Analogously to any tailless aircraft, the side-by-side twin is sensitive to CG in vertical mode and trim must be closely maintained. Although not ducted, the Boeing V-22 Osprey convertiplane suffers from this issue. It is not often an operational limitation, but it does need crew/staff training and careful management.
Similarly the quad arrangement brings the same benefits as the tandem wing in allowing a wide range of CG positions. However the quadrotor does not scale well to full-size aircraft, as discussed at Why haven't quadcopters been scaled up yet?


text:
Could Apple Airtags interfere with aircraft communication and navigation systems?
German media reports that Lufthansa has banned Airtags in checked bags:

This is specifically because of the transmission function.

Lufthansa claims that the transmission function needs to be turned off during flight when in checked luggage, just as is required for cell phones, laptops, etc.

Factoring in frequencies, transmission power (and any other relevant factors), it is possible Airtags could interfere with aircraft navigation and communication systems? Given other airlines haven't banned them, it is curious why only one would choose to do so.

They’re low-power Bluetooth devices, similar to earbuds.  Complete non-issue.

Surely it has nothing to do with people using air tags to track their luggage which Lufthansa lost and cannot find.

No, the Air Tags will not interfere with aircraft avionics and communication systems. In response to Lufthansa's ban, Apple stated that its Air Tags are compliant with all aviation safety regulations, and the statement was supported by both the FAA and NTSB in the United States and in Europe, the EASA stated it does not ban or allow the device, it is up to each airline to decide. Since Lufthansa has backtracked on its ban and again allows Air Tags in checked bags citing an ICAO guideline for issuing the ban in the first place.

Air Tags use 2.4GHz (Bluetooth-LE), 13.56MHz (NFC), and 6.5-8GHz (Ultra-Wideband). These are the same frequencies as any modern cellphone.

@DavidKryzaniak And to what extent is an airliner's fuselage a resonant cavity?


text:
How will the update from DO-260A to DO-260B improve safety and efficiency in air traffic management significantly?
From what I know, the version update from DO-260A and DO-260B offers new features such as on-ground vs airborne source selection logic, ADS-B failure annunciation, the split of the quality indicators between horizontal and vertical component, the addition of GVA and SDA, etc. In comparison with the previous versions, it is expected that the latest version could at least increase efficiency and safety in utilizing airspace.
Assuming country “X” has implemented DO-260A as their minimum standards for its surveillance infrastructure and it could accommodate up to 1000 planes every hour. Then it decided to update to DO-260B and could accommodate up to 2000 planes every hour.  And yet I ponder with all of these new advancements in DO-260B, is there:

A way that I can estimate and measure quantitatively based on the update? (Assuming that DO-260B does improve safety and efficiency in air traffic management)
With the quality indicators in DO-260B, how are those used in the process of that estimation?
A website that analyzes safety and efficiency quantitatively on air traffic management, specifically after changing MOPS such as from DO-260A to DO-260B ?
 Thank you in advance.

Source:
1. https://atl.aero/wp-content/uploads/2019/01/ADS-B-Out-Instalaltion.pdf
2. RTCA DO-260B DRAFT Version 4.2


text:
Is RTCA DO-260 Change 1 the same as DO-260A?
I am a bit confused as to the history and the naming for 260 documents. This is the timeline I perceived.
From earliest to latest:

DO-260                  ( September 13, 2000 )
DO-260 Change 1         ( June      27, 2006 )
DO-260B                 ( December  2,  2009 )
DO-260B Corrigendum 1   ( December  13, 2011 )
DO-260B Corrigendum 1   ( December  13, 2011 ).
This one says it is supplemental to DO-260B for corrections in regards about typographical errors, etc.

Am I missing something? Please correct me if I'm wrong.

At the moment I assume DO-260A is the same thing as DO-260 Change 1 yes? Another is what are the major differences between DO-260B and DO-260B Corrigendum 1 ? Thank you in advance.

Sources:
RTCA store website,
Fourth Meeting of APIRG_ICAO

DO-260 change 1 is not DO-260A.

DO-260A released in April 2003
DO-260A change 1 in June 2006
DO-260A change 2 in December 2006

The releases marked as 'change' contain editorial changes, clarifications and corrections, but are effectively the same specification.
The alphabetical versioning is for subsequent releases of a new specification of the same system. In these releases the specification is changed, i.e. the behaviour of the system specified is different.

Thank you! Also, would you happen to know why RTCA used the number "260?" Well...numbering documents in general really like DO-242, DO-248, etc. What are their reasons for using those numbers ?

@AghaKusuma the numbers are assigned sequentially. If a RTCA Special Committee wants to publish a new specification, the document will be assigned the next available number. Note that the time between the number assignment and the actual date of publication varies, and so the numbers are published sometimes slightly out of order.

Also, the Corrigendum is supplemental information, specifically: "This document is a supplement to DO-260B. It contains corrections to typographical errors, corrections to test procedures and notes added to clarify existing text. These changes represent feedback from industry use of the published version of DO-260B. This supplement does not alter any existing requirements and does not affect the compliance of any equipment designed to meet the original published document."

A clarification on the DO numbers.  When industry or the regulatory agencies determine the need for a specification, plan, guidance, or other document; the request is submitted to the RTCA Program Management Committee (PMC). The PMC will validate the need and publish "Terms of Reference" which defines the document (with number) and assigns the task of generating the document to a Special Committee (SC).


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Why are high-bypass turbofans faster than turboprops?
The turboprop, and especially the high-blade-count propfan, differs from the high-bypass turbofan principally in throwing away the draggy outer ducting. This makes it easier to obtain a greater diameter, moving more air slower, and hence higher efficiency. Fewer blades further increase aerodynamic efficiency. Tip speeds remain much the same, limited by the speed of sound. So why are turboprops significantly slower than high-bypass turbofans?

Tip speeds do not actually stay the same, turbofan (and turbojet) blade tip speeds can exceed the speed of sound, up to Mach 1.3 (actually stated M1.4 in the link, my experience is M1.3 but also my professor used to mention M1.4) as seen in link. As they are enclosed, the negative effects of the shock are much smaller than if it were an open propeller. Open propellers (and also propfans) are also affected by aeroelastic effects in transonic speeds which are very hard to isolate with their high span-to-chord ratios (I don't like calling it aspect ratio for propellers). Not to mention the NOISE that an open propeller generates at transonic speeds.
Additionally, due to the fan being followed by a stator (which uses the rotational component of velocity added to the airflow by the rotor) and the converging fan duct geometry, turbofans accelerate the flow further than propellers do. The exhaust from the turbine is also faster than the fan and constitutes a greater fraction of the air ingested by the engine, which increases the average exhaust velocity of the engine.
Imagine a turboprop flying at Mach 0.8 and the propeller tip speed being limited to Mach 0.9 . This requires the propeller tip to have a tangential velocity of M0.41 (Pythagorean sum). On the other hand, at the same flight speed, a fan blade tip limited to M1.3 can still have a supersonic tangential velocity of M1.024 . It is obvious that a fan blade turning more than twice as fast can exert greater forces on the airflow.
The thrust is generated by accelerating airflow. We can either accelerate a lot of air by a little bit (think of a helicopter rotor) or a little air by a lot (afterburning turbojet). A relatively slow-moving propeller blade (due to the reasons above) can do little to accelerate airflow going through it, relative to a fan that turns much faster in an enclosure.

Ah, so fan blades can go supersonic. That led me to an earlier question, which mine to a large extent duplicates: https://aviation.stackexchange.com/questions/22316/why-do-turbofan-blade-tips-exceed-the-speed-of-sound-while-prop-tips-should-not Note that exit velocities and efficiency are not related to my question here.

exit velocities are relevant, I was trying to elaborate how turbofans can accelerate airflow at M0.8 while a propeller can't.

If you want to know what happens if you try to make a turboprop do jet speeds, Tupolev tried that; suffice it to say, from all I have read, the results were un-bear-ably loud

I see what you did there. The NK-12 is a beast.


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Does the Air Force (United States) help design their own planes?
I know that companies such as Lockheed Martin and Boeing design and build planes for the US Air Force. However, when I was thinking about this, I realized the Air force has no part in designing the thousands of aircraft they own (or am I wrong). Does the Air force basically act as a consumer or do they help in the process of designing the jets? (I know there is a job of an aircraft mechanic but they act as maintenance people and don't design the jet.)

Perhaps some useful language is that the A.F. "directs" the entire process.  The AF doesn't sit down and use Photoshop to make a jpeg to send to a supplier in China to fabricate a hinge or fan blade, but the A.F. "specifies" or "directs" the whole show.

I suppose this depends largely on how broadly you define "help design." If you're limiting the scope to an actual aircraft design project, the other answers cover that well: the USAF determines what it needs, Congress changes it to something that brings business to their districts and approves funding for it, and then the USAF issues detailed requests for proposals from the likes of Boeing, Lockheed Martin, Northrop Grumman, etc.
However, the Air Force does do a ton of research that develops a lot of the technologies that will eventually be included in those airplanes. It also does a ton of verification testing, as well as overseeing development contracts. This is pretty much the entire mission of the Air Force Materiel Command, which employs approximately 80,000 people. AFMC is the most-funded major command within the Air Force, representing 31% of USAF outlays, according to Wikipedia.
The Air Force Research Laboratory oversees early-stage research projects, as well as conducting a good deal of its own research that will eventually end up in USAF hardware, though it typically doesn't design or produce the actual production hardware itself.
Bases like Eglin, Edwards, Arnold, Holloman, and others perform aerodynamics testing and flight testing for experimental aircraft as well as for production aircraft and engines for all branches of the military. For example, Arnold conducts most of the wind tunnel testing for U.S. military aviation, as well as testing jet engines, rocket motors, and space vehicles. While their mission is primarily military, they also do a lot of contract testing for civilian projects, including commercial projects as well as NASA projects. There's a good chance that the engine powering your airliner was tested there at some point during its development.
So, if your definition of "help design" includes conducting a lot of the early research that ultimately makes the production technologies possible and design verification testing (including flight testing, wind tunnel testing, engine testing, etc.,) then, yes, the USAF does participate quite heavily in developing its aircraft. However, if you're excluding these activities and also excluding developing the detailed requests for proposals, overseeing the bidding, overseeing the development efforts of the winning bidder, etc., then you could argue that it doesn't "help design" them to a large extent. The latter would seem to be a short-sighted view of what "help design" means in my personal opinion as an engineer, though.

Thanks! Did you work for any of the companies stated above?

@LukeJustin No, but I worked for the Aerospace Testing Alliance at Arnold AFB.

Did you test any types of planes or jet engines. Did you see any planes before they were released to the public?

@LukeJustin I wasn't testing them personally, unfortunately. I was just there for a few months on a summer engineering internship during college. I did see some of the tests in progress, but I wouldn't be allowed to talk about exactly what (it was a USAF base, after all.)

While the Air Force (or other military unit) does not design their own jets, they do release specifications and have contests (e.g. the ATF that selected the F-22). This is basically the way it works (and not just for fighter procurement):

A design specification is released, these include things like range, mission, weapons.
A "paper" design is submitted from companies like Boeing and Lockheed Martin. These may be based off of existing designs or "clean sheet" designs.
The military funds the development of one or two prototype aircraft from a number of paper designs.
The military evaluates each design against the specification and selects a "winner".
A larger contract is awarded

So it's not really up to companies like Boeing or Lockheed Martin to come up with their own designs and sell that to the military. They work under strict program requirements to develop something "to spec" and then demonstrate the capability. The military has a hand in the design process as this goes along, so they are not entirely "out of the loop".
The secondary market is different though. Boeing may sell (with the approval of the US Government) these aircraft to certain foreign military groups. These work more like selecting something from available stock in the dealer lot.

This has changed over the years. There's an anecdote that the specification of the original Hercules fit on a single page; by the time the C-17 rolled around, its spec was so extensive it filled up a Herc's cargo bay.

@Hobbes I think that's a joke, not an anecdote. ;)

I propose to add a point on top of the list mentioning that military planners study political, technical and social developments and come up with scenarios for future conflicts. From that, they derive what follows in the first point of your list. This is substantial and very influential work because here the path the design will take is determined.

@PeterKämpf A very valid point. Studies by Defense Acquisition University have shown that decisions in the first 6 to 12 months of a program will drive 80% of the life cycle costs of the program. The next 10 years or so is just working out the details. The military is concerned about what it does. The contractor has to work out the how. But much of an aircraft is mature technology. The 'new' tech that provide the unique capabilities is a constant discussion between the military and the contractor. It not only has to perform as desired, it has to be sustainable.

@PeterKämpf That's one theory of how the political process might go, but in reality there are other just as valid theories (which I personally think have more merit). For an aviation site, the starting point of this answer is completely appropriate.

The answer to your question largely depends on your definition of "design", as well as the specifics of the procurement program.
In general, the Department of Defense (DOD) does not have engineering authority over the equipment it procures. That is, Pentagon employees do not produce and sign engineering drawings for hardware built by contractors. However, almost all such data is reviewed by the DOD in detail before being implemented.
More broadly, though, the final configuration of a military aircraft is the result of a continuous back-and-forth between the supplier and the DOD. In my experience, this interface occasionally borders on collaboration. Sometimes a Pentagon requirement is broad and it is expected that the contractor use its expertise to come up with a creative implementation independently. Sometimes the DOD uses a so-called "directed solution", which tells the contractor almost exactly what the final product should look like in practice. And everything in between.
Your parenthetical actually illustrates a very important example. I spent years working on teams tasked with figuring out how to design military aircraft to be as friendly as possible to the mechanics and maintainers. Without the input of DOD mechanics in the field, we could not have done our job well, and the final design of the systems was certainly different than it would have been without that input.

This is the correct answer, the question simply depends on your definition of "design".

@Peter Schilling What company did you work for?

In short: the US Air Force does not do any design work, primarily because they have deliberately decided not to maintain the necessary expertise. Other DoD entities do, to a greater or lesser extent. This has been true for the past couple decades, but was not true historically.
As other answers have stated though, a detailed answer depends what you consider "design," "aircraft," "does," and "help" to mean.
"Aircraft" is probably the easiest: it is fairly common to require specific systems be put on a platform. For example, you might see a requirement for a specific IFF transponder (or the prime contractor might select a specific IFF transponder), which might be provided as Government Furnished Equipment (GFE - essentially the government is supplying their own parts to be installed). That box is then part of the design of the aircraft, and the prime contractor is relying on the government to ensure it works as required. Small avionics boxes like that are more likely to have more design influence from the government side. So, if exerting design influence on that box counts as exerting design influence on the "aircraft," then the USAF does do design work. It is overwhelmingly common to have a random assortment of configuration items (especially avionics) provided as GFE, usually as a misguided attempt to save money.
"Does" is next. The USAF got out of the design business. I am not aware of any specific policy decision that drove this, but anecdotally it seems to have happened in the '90s. Before that, it was much more hit or miss. E.g. the SR-71 was built based on a half-page of requirements while Lockheed stiff-armed the USAF away from any design insight, But John Boyd famously drove the overall capability requirement for lightweight fighters and exerted tremendous design influence on the F-15.
"Design" and "helps" are a bit sticky. At a 50,000 foot view, the idealized modern US acquisition process goes something like this: combatant commanders (the heads of CENTCOM, PACOM, etc.) plan constantly for campaigns they might need to execute. As part of that, they note gaps in the capabilities available. E.g. maybe PACOM is concerned about pirates, and wants a capability to conduct small boat boarding operations from fast-mover aircraft. That capability need gets rounded with all the others and sent up to the Pentagon, where the Joint Requirements Oversight Council figures out how to address it (see: DOTMILPF), and racks-and-stacks it into a package of capabilities. If that package has a materiel component (i.e. if you need to buy stuff, not just re-train or whatever), it goes into the budget request and gets money allocated to it. Then the money and capability requirements go to one of the acquisition commands (AF Materiel Command, IIRC), which takes that crazy wishlist, turns it into a specification (usually. USAF does much less specifying than other branches), and goes and buys the thing specified. As part of the "goes and buys" process, the government usually takes control of various design elements after they are complete. The degree of control and degree of insight the government has varies wildly from program to program, but the AF tends to rely on prime contractors much more than the other services. Finally for any major weapon system and many other systems, there is an "operational evaluation," where real users really test it out to see that it meets the capability needs that JROC described. The feedback from that might well influence design.


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Does the F-16 have a movable inlet?
Does the F-16 have a movable inlet? Everything else like the f15 has it does the f16?

No it doesn't, and not everything else has movable inlet.

The F-18, F-22, F-35, and Rafale don't have them either. That's because they represent added complexity (more moving parts means more risk of failure), added cost, and added mass, for performances gains that are not worth it anymore. High supersonic flight speeds are no longer required in today's missions. Flying fast was mostly useful to fly away from combat or intercept incoming threats. Missiles have become so fast that you can not outrun them anyway, and radar detection range has increased to the point that you can detect enemy planes sooner, thus not needing Mach 2.5 interceptors. And if you're flying at Mach 1.5, a simple inlet will have some small losses, but you would be better off with a bigger engine than a variable geometry inlet.

On the other hand, the F-15, Concorde, and SR-71 had movable inlets. Concorde could supercruise at Mach 2.2, the SR-71 was able to reach Mach 3.2. Those are speeds where pressure losses in front of a simple inlet would be enormous due to the shock waves, totally ruining your engine efficiency. The F-15 inlet has two moving plates allowing for 3 weak shocks thus improving greatly the performances at Mach 2.5, with the inlet efficiency going from 0.5 to approximately 0.85 with the addition of the moving plates. As you can see from the diagram below, the difference in efficiency is only meaningful above Mach 1.5. The F-16's requirement was good performances for low price between Mach 0.6 and Mach 1.5 and that's why they did not use variable geometry inlet.

No it does not. Simple oval shaped tube.

{Image from Wikimedia}

Hi, why does it have a vertical bar at the front of the intake?

Structural support, heater, and CCD (Crew Chief Divider).
Provides a bit of strength to the opening, prevents ice from building up on it, and prevents a crew chief from going down the pipe in one piece.


text:
What would an inlet look like on a hypersonic aircraft? Would it be fixed or movable?
What would an inlet look like on a hypersonic aircraft? Would it be rectangular or like a cone (assymetric). Would it move and if so how?

If your aircraft is made to fly at only one speed (like the X-43) there is no need to have a variable inlet. Optimizing it for only one speed is enough.

I dont undertsand. Wouldnt you need a movable inlet to compensate for the shockwave changing position

If your speed don't change, the shockwave position won't change. For example, the X-51 is not design to fly slower than mach 4.5 nor faster than mach 5.1. This speed range is really narrow, thus no need to design an inlet efficient from full stop to high hypersonic speed.

But doesnt the speed change as you speed up from 0 to your normal speed?

@LukeJustin, you can't use the same engine across that range anyway. Turbines are useless above around M3, ramjets start at maybe M0.5, are efficient above M2 or so and top at M6 and scramjets start above M3 and only become efficient above M6 or so. Much more reconfiguration than just the inlet.

@ManuH You said if you speed dont change shcokwave wont change position. But what about the speeds to get up to Mach 5?

@LukeJustin The X-51 is accelerate to mach 4.5 with a ditchable rocket.

Also note that all ramjet/scramjets are accelerated via external means (either drop from a mothership like the Leduc 0.10, or most impressive the D-21) another engine (such as the nord 1500 griffon) or a rocket (such as the X-43).

I think you may review your question because hypersonic inlet do exist but your question can easily be transformed to include engines combining turbojet engin and a kind of ramjet (e.g. the engine of the Nord 1500 Griffon II or the J58 with its by-pass mechanism).


text:
Are there any hypersonic planes in service today?
Are there any hypersonic (Mach 5+) planes in service or officially being built? Or all the talk I here about the SR-72 rumor. Thanks!

I was thinking of the scaled composites model 339 space ship two but its max speed is between Mach3 and Mach 4 (not hypersonic). I'm still searching but I think this is the fastest passenger plane "officially being built" right now (OK, it is still in test phase in summer 2020, but I really hope it will perform commercial flights soon)

As far as supersonic commercial flight goes, Nope. The Concorde and the Tupolev Tu-144 are both long retired.
As pointed out in the comments, hypersonic commercial flight has never existed.
The SR-72 you speak of is supposed to be flying by 2025.
This article points out that

Only a few experimental planes have ever reached hypersonic speed, and the last time a hypersonic plane flew with a person on board was in 1967. That plane, the North American X-15, was powered by rocket engines, which are impractical for commercial air travel.

The same article mentions a potential 'in-development' aircraft:

The Stratofly consortium plans to begin wind tunnel tests of the engine components. But it will take many years to bring Stratofly to life. The key technologies might not be ready until 2035, Viola says, with another decade needed before flight tests are completed and the plane is ready to carry passengers.

Were the Concorde and Tu-144 hyper sonic or just super sonic? I thought they only went Mach 2ish, not 5+

Shuttle routinely flew hypersonic on entry.

@OrganicMarble I totally agree but it is not "in service or officially being built" anymore.

Right now none are routinely flying and only two aircraft, the experimental X-43 and the X-15 ever made powered hypersonic flights successfully.
Hermeus Corporation has a Mach 5 ramjet-powered business jet under development. Lockheed Martin has been developing a Mach 6 aircraft under the designation SR-72 and has stated the real aircraft could be flyable by 2025.  And the X-30 National Aerospace plane made simulated flight of fancy some thirty years ago.
Hypersonic weapons have made great strides in the form of surface to surface and surface to air missiles as well as EM propelled projectiles ie ‘railguns’ capable of firing projectiles at speeds ranging from Mach 7 to Mach 13 and offering greater kinetic smash at the target than a chemically propelled round with a conventional explosive warhead.
All of these aircraft still remain fantastic ideas and cool airbrush art or CGI concepts.  Out of all the concepts outs there, I’d venture a guess that the LMACO SkunkWorks concept aircraft has the greatest chance.  There still remains a colossal amount of engineering needed to get craft like this flying successfully.  I’d suggest reading Ben Rich’s book about the Skunk Works and the struggles they went through in the 60s to perfect the A-12/SR-71 Blackbird, which was fairly tame compared to the thermal heating problems and novel propulsion system designs like SABER or SCRAMJETs.

Hypersonic missiles are already common - many SAM and AAM operate in the Mach 4 - Mach 6 range. The big distinction would be an airbreathing hypersonic vehicle.

@Therac You mean like the meteor which is an hypersonic air breathing vehicle and definitely not a plane.

@ManuH On the edge. It's air-augmented, but I'd err towards not calling an engine air-breathing unless the air is used as an oxidizer. The Meteor uses air as an optional added working mass; the fuel has its own oxidizer.


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How did the B2 Bomber achieve yaw

I came across this diagram of the B2. I was wondering how it achieved yaw. If I'm correct usually the rudder on the tail would do this, but the B2 does not have a tail. Does it have no need for yaw, or does something else act as the tail?

It uses differential air brakes.

They use differential breaking. The elevons are split, and can open up both above and below the wing to increase drag on one side, yawing the plane.


text:
How are horsepower and standard displacement for a rigid airship calculated according to this book?
This is a follow-up based on a question posted here. I updated my initial question but came to the conclusion that a new post might be more manageable. For bibliographic reference(s) and/or getting a copy please refer to the linked question. Be that as it may, this is my current issue:
Having had some time at hand, I went back to the book. The solution of the problem set above continues like this:

Assuming the weight of the power plant and its cars to  be 8.lbs/Hp. and the weight
of the fuel and the fuel system to be .6 lb. per horsepower hour, the total weight
of power plant, fuel system and fuel is

[8 + ( .6 x 60 ) ] Hp. = 44 (Hp.)

combining this with the military load,

15,000 + 44 (Hp.) = .357

No issue so far, I was also able to compute this formula:

However, I cannot make heads or tails of the next part

Combining equations (1) and (2)

My question is how did the author arrive at equation (3)? Where does the value 48D(2/3) come from and how were these formulae combined.
When using the previously computed value of .0357 for D, I am not getting the result of 42,000.
Apart from the "actual" math involved, I fail to see the meaning of the components involved in equation (3):
Could someone help me out here and maybe translate that formula into English sentences?

The book author just eliminated Hp from 1 by the formula 2.
$$ 15000 + 44(.39D^{2/3}) = .357 D$$
Do arthitmatic and rearrange:
$$.357 D -17.16 D^{2/3} = 42000$$
Divide by the .357 to get
$$ D- 48.067 D^{2/3} = 42016.8 $$
Then round off because he used a slide rule not a calculator.

Dividing your second line by 0.357 should increase  the 42,000 value on the righthand side by roughly a factor of 3, rather than they very slight change shown in the 3rd line. Is 42,016.8 really what you intended there? Or is 42k not really what you wanted in the 2nd line (15k, perhaps)?

@RalphJ I'll surely take your comment into account. At the moment, my goal is merely to understand the equations and how the author arrived at some of the results. The present question is a actually a good example for some "missing" steps in solving the  equation, at least in my opinion, since I haven't done any "sophisticated" maths for quite a while. For the time being, I'll stick with the values taken from the book Appreciate the additional details.

My mistake in copying notes. The 42000 in second equation should be 15000 (from eq 1). Then divide by .357 gives 42016.8.  Just for fun, I figured out that D=213600 (approximately).


text:
Are AWOS data open for commercial use?
I 've found out this great website where you can get historical AWOS weather data. I would like to use it for a commercial application but I don't find any data license. Do you know if the AWOS data are free to use for commercial purposes? Can you point me to a relevant link?

A bunch of random folks on the internet aren't going to know that. I'd recommend that you contact the fine folks at Iowa State and ask them.

Your website source from Iowa State University may or may not be open for commercial use. As Freeman has said in his comment, it would be difficult for anyone here on the internet not affiliated with the university to know. Your best opportunity to use their data is to contact the university directly. Their contact information is at the bottom of the link you provided.
Otherwise, your best option is to contact NOAA, the NCEP or the National Weather Service (basically, one and the same organization). They collect, consolidate, and archive the data at their source. That is where many commercial applications get their data.

Thanks for the reply. Really helpful. I was maybe thinking that it was commonly known in the aviation community if this data source is open or not, hence my question. Your suggestions and the one from FreeMan are very good, I will directly contact the organizations that you mentioned... and post the answer here once I get a reply :)

@greghor - No problem. Unless Iowa State has done something to add a unique value added proposition to their data in order to make the user interface easier for commercial use, it might be better to go directly to the source of the information. After all Iowa State is getting their data (probably for free) from the NCEI, a division of NOAA. More than likely, so can you. Good luck.

+1 on Dean’s last comment.  This data is produced using taxpayer dollars, and is broadcast for public consumption.  You are wise to ask these questions, especially if another entity has gone to the trouble to collect and archive it, and especially if you intend to use it commercially, but you ought to be able to find a way to gather and use what is in the public domain.

The question has been closed but let me write here the answer I got from NOAA. In the general case, AWOS data can NOT be used for commercial purpose


text:
Does DA/DH refers to ceiling?
If airport is reporting FEW001 or SCT001 and RVR is above 600m, does it means that there will be no LVO because ceiling is defined as more than 4/8 and DA/DH will be more than 100ft because DA/DH is defined in the name of ceiling.
Or what if some of the SCT001 clouds will get over missed approach point, which is exatly at DA/DH, when pilots have to make decision. So they might not see the RWY at DH200ft, because of SCT001.

The structure of your question is a little confusing. Are you able to edit it to provide more clarity. That will increase your chances of getting the answer you seek.

If I am understanding your question correctly:
An LVO is Low Visibility Operations which is defined by the FAA in AIM 2-3-4 and 4-3-19 as well as Advisory Circular 120-57a & ICAO OPS Low Visibility Operations (LVO)

Low visibility operations are those that occur when the runway visible range (RVR) is below 1200 feet (360m).

So, your RVR above 600m would not indicate the need for LVO.
A Ceiling is defined by the FAA

Ceiling means the height above the earth’s surface of the lowest layer of clouds or obscuring phenomena that is reported as “broken,” “overcast,” or “obscuration,” and not classified as “thin” or “partial.”

And, Broken (BKN) cloud layer is defined in AIM 7-1-31

9. Sky Condition.
The sky condition as reported in METAR represents a significant change from the way sky condition is currently reported. In METAR, sky condition is reported in the format:
Amount/Height/(Type) or Indefinite Ceiling/Height
(a) Amount. The amount of sky cover is reported in eighths of sky cover, using the contractions:
SKC clear (no clouds)
FEW >0 to 2/8
SCT scattered (3/8s to 4/8s of clouds)
BKN broken (5/8s to 7/8s of clouds)
OVC overcast (8/8s clouds)
CB Cumulonimbus when present
TCU Towering cumulus when present
Note 1: “SKC” will be reported at manual stations. “CLR” will be used at automated stations when no clouds below 12,000 feet are reported.
Note 2: A ceiling layer is not designated in the METAR code. For aviation purposes, the ceiling is the lowest broken or overcast layer, or vertical visibility into an obscuration. Also there is no provision for reporting thin layers in the METAR code. When clouds are thin, that layer must be reported as if it were opaque.

So, the cloud layers you described as FEW001 or SCT001 would not be considered a ceiling. And, they would not be considered IMC.
More insight on what can be done at the MAP can be gathered from this question:
Missed Approach below DA/DH

You’re right if clouds are less than 5/8 they do not construe a ceiling and does not satisfy the ceiling being below DA/DH.
DA/DH is as the name applies is the decision point for the pilot. If pilot believes a safe landing is not possible at DA then she/he need to do a missed approach. That’s is not limited to clouds which is less than 4/8 but also can be (for the lack of imagination) say a burning car’s smoke which is precluding the visual with the runway.  There are 3 requirement to meet before you can descent below DA/MDA per 14 CFR § 91.175:

(1) The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers, and for operations conducted under part 121 or part 135 unless that descent rate will allow touchdown to occur within the touchdown zone of the runway of intended landing;

(2) The flight visibility is not less than the visibility prescribed in the standard instrument approach being used; and

(3) Except for a Category II or Category III approach where any necessary visual reference requirements are specified by the Administrator, at least one of the runway environments is distinctly visible and identifiable to the pilot”

First paragraph addresses your question. You need to be in a continuous safe position all the way down to the runway. So if something is blocking your vision to the runway then you need to start a missed approach.
On the other hand, if you’re flying Part 91, you can shoot the approach even if the weather is below minimums (e.g. more than 4/8 clouds reported below DA/MDA). In that case, if you satisfy the 3 requirements above at the DA/MDA, then you can continue  with the landing. Part 121 and 135 don’t even allow to shoot the approach in the first place, in such a case.


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What are the laws on photographing aircraft in Australia?
Are there any types of aircraft that I am forbidden from photographing aircraft. (Australian aircraft, primarily) in Australia? Are there regulations on what I do with the photographs I take? I assume there is some sort of thing published by the government but I couldn't find any.

Is this on-topic?

This might be better asked on Law Stack Exchange since you are asking about regular laws rather than aviation regulations.

There is no law in Australia prohibiting the photography of aircraft. I have been approached by police while doing it just to see what I was up to, but after a polite exchange they were happy and moved on.
Potentially, if you're on private property (eg an airport terminal), the conditions of entry are allowed to prohibit the taking of any photos. But I'd be shocked if that is applied anywhere outside of theatres or defence bases. You try telling travellers they're not allowed to take an airport selfie for instagram.
If you're using the photos for commercial reasons, and they include a trademark (like an airline logo), you might need to contact them for permission, I'm not certain how that process works. But this certainly doesn't apply for private use.
Having said all that, I am not a lawyer, so if you're worried or want specific information you should at least ask on a legal forum, if not pay for a professional opinion.

I haven't heard of problems when taking photographs on land. Note that if you are flying and you stray into an "Air Defense Identification Zone" anything that takes a photo may be confiscated after landing; assuming you aren't shot down!
https://vfrg.casa.gov.au/operations/air-defence-identification-zone/general-2/


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Why do the engine nacelles on the P2V Neptune extend so far back?
I was studying a picture of a P2V Neptune seen from below, and what struck me was how far back the engine nacelles reach. Unfortunately, I can't find that original photo but this one shows what I'm talking about:

Image source
Unlike all the other piston-powered planes I've seen, the engine nacelles actually extend past (what I presume to be) the flap hinge line, ending behind the trailing edge of the wing. There is a noticeable gap in the nacelle to allow the rear part to rotate with the flap. Since the relatively flat radial engines don't even reach back to the leading edge, and the landing gear are situated far forward from the back of the nacelle, I don't see what advantage these elongated nacelles give. I doubt they help reduce drag, as they add surface area and don't end smoothly.

The engine nacelles on the B-25 also extend past the trailing edge.

I can't speak to the design criteria, as I wasn't there when the airplane was designed, but I do have time in type and experience flying and maintaining the P2V.
There's more going on than just engine, with respect to the nacelle.  Remember that the nacelle is partially a fairing for the engine, and serves as the aerodynamic afterbody, but that conical shape also serves to fair additional structure, including the engine mount, oil tank, and in this case, the landing gear.  Notice that the structure is greater beneath the wing than above, because the gear resides within.  The angle of the nacelle afterbody is more than just the engine nacelle; it's also fairing the gear.  If you follow the lines on the lower surface, after the gear doors are shut, you'll follow a consistent arc which actually takes the nacelle aft beyond the straight trailing edge of the flap.  The upper surface, coming over the top of the wing, does join the wing at the trailing edge, but continues to mate with the lower lines, which extend further.
The simple answer may be the reason is the landing gear (and associated hydraulics, actuators, etc.  There's also more structure and equipment contained within the afterbody and along the rear spar.
That said, as radials go, the R-3350, and the nacelle on the P2V, is quite large, and features integral air intake, rather than an external scoop as some engines have used.  It's large enough that it opens in three sections, to work on the engine.  It's very well designed, and represented the most advanced that radial technology reached.
The P2V was advanced, for its time, but complicated, hydraulically.
Those two jets on the wings, J-34's, avgas-burning turbojets, tripled fuel consumption when lit off, and of several different door fairings designed, turned out to be somewhat unreliable.
It's one of the few airplanes I've flown that required a crew member in the back to cut safety wire on the hydraulic lines at the varicam (variable camber horizontal stabilizer) and simultaneously swap valves on the main and sub-main hydraulic systems, at the same time the pilot in the cockpit swapped the control at the back of the pedestle in the cockpit.  Not a big problem with a crew of 11, but in modern times, with a crew of two, a potentially big, big problem.
Off-topic, I know.  The airplane was a workhorse.

Amazing facts - thanks!


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How do fixed Vs rotary wing licences work in Britain?
I am eager to be able to fly both helicopters and fixed wing planes commercially. Would this require two separate Commercial Pilots Licenses or is there some amount of overlap? For example, can you do a fixed wing CPL then upgrade it to a fixed + rotary CPL? How does it work?
I live in England, so are there differences across international borders or is this pretty standardised?

Rotary and fixed wing are different licenses in the UK, whether you are private, commercial or ATPL. You first get a PPL, then a CPL for the type of aircraft, and you must pass the test for each separately. The hours requirements are reduce for each if you have the other aircraft type license, for instance for the CPL-H you can count 50 hours of PIC time if you have a CPL-A, for example, and there are reduced cross-country navigation requirements for the PPL as you've already shown you can navigate from getting the other license. So some training and experience carries over.
EASA countries work on a similar model, the exact requirements vary from member state to member state.

Thanks! I have lots of work to do then!


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Why do tail dragging seaplanes not exist?
I am in the process of starting to build an ultralight that will be a tail dragger with floats, and in my conversations with the fellows I'm building it with, we've realized we've never seen a plane like it before. Does this mean we are doing something dumb? Thanks in advance.

Maybe for floats to be sufficiently buoyant they have to be so big that they can also be statically stable on their own (like having four main wheels and no tailwheel) so there is no need to tail drag, whereas on land you can save weight and just use two main wheels (or skis, but those are still much smaller than floats) but then you need a tail wheel to balance.

Welcome to aviation.SE! Where the wheels are isn't relevant to an aircraft on floats so I'm not sure what you're asking. There are lots of J-3s and other 'tailwheels' on floats. There's even a DC-3 out there.

@Pondlife I think the OP is not referring to tail draggers that have been converted to have floats, but planes that literally drag something at the tail through the water.

@DKNguyen Thanks, that does make sense and John's answer describes exactly that

Taildraggers are susceptible to ground loops. That’s bad enough on land, but much worse on water.

It's been tried before (there isn't all that much in aviation that hadn't already been tried by WWII).  The tail float means the aircraft CG is aft of the hydrodynamic center of the main floats, and like a tailwheel airplane, the machine will be dynamically unstable, steering wise, when on the step, same as a wheeled taildragger and you will have to prevent it from switching ends with your feet the entire time it's on the water moving.
It could be a real handful to take off and land, especially when there are waves and gusty winds (and seaplanes do have to cope with crosswinds from time to time).  You have to make the rear float steerable, and you have to give it enough lateral bite in the  water to have good leverage in steering the thing when below the speed that the air rudder is working.  I don't think there is any drag advantage to 3 smaller floats vs 2 larger ones.
Traildragger advantages, like prop clearance, aren't that much of a benefit here, and since the conventional float configuration is so much easier to control, why bother with a taildragger float setup?
Ideas come along for new configurations all the time, but if you dig enough you find that somewhere in the distant past it's already been tried, and while it may have been functional, the "old" conventional way that everybody uses has been around so long because it's simply what works best.

So it kind of begs the question...why not take the same approach on land? Why is prop clearance not a problem on float planes anyways?

When you start the takeoff run on a normal floatplane, you drive the nose up high with full aft elevator at the start until you start to come up on the step.  So there isn't really the need to have the nose pointing up all the time.  Also the level attitude when displacement taxiing makes you much less susceptible to winds.  Tail low slow taxiing on the water adds a whole new flavour to the hazards.  Water impingement on the  prop on seaplanes (which does a lot of damage over time) is mostly a problem of taking off in significant chop when you are just getting started.

On the airplane pictured, it looks like a wave could rip the rudder right off. Eeeek!

@DKNuygen The floats have to be big anyway, and because they're long the CoG nearly always lies within their footprint.  You could, if you wanted, have a landplane with four wheels (roughly where the ends of the floats would be) on long legs which would sit almost horizontal at rest.  But why bother? It'd be heavier than it needs to be.

authors: Bob Jarvis - Слава Україні


text:
What do pilots landing on aircraft carriers do when the ship is not visible due to weather?
Say an aircraft takes off of a carrier to do a mission somewhere, and upon returning to the carrier the pilot comes to find that a thick layer of fog has formed just above the surface of the water in addition to low-level clouds. The pilot may have the ship's exact location, but is it still possible to land?
In the civilian world, usually IFR minimums are 200 ft. AGL, but I am assuming this doesn't apply to military aircraft out at sea, mostly because the ocean is usually a flat surface at a known altitude, (0 ft. MSL), and therefore the risks of terrain other than the surface of the ocean would not be a factor. I also assume they don't apply to carrier-aircraft because, well, they're the military.
However, if the ship is completely shrouded by weather, what would the pilot do? Are there instruments/procedures used to execute a super precise spot-landing with virtually no visibility? What if the plane is bingo fuel?

https://en.wikipedia.org/wiki/Modern_United_States_Navy_carrier_air_operations#Approach

Carriers often(?) operate as part of a fleet of other ships, although their heights are pretty low.

One big advantage of a carrier over normal airports is that the average airport can't just move 20 kilometer to a less cloudy spot.

When both the aircraft and airport are correctly equipped, even airliners can land in thick fog. See ILS, especially CAT III, and Autoland. Without that some major airports like CDG would be closed a bit too often!

Improvise. https://www.forces.net/services/tri-service/day-royal-navy-pilot-landed-spanish-container-ship

My experience is circa 1990s, but I can offer some perspective on US fixed wing operations.
Besides TACAN and ASR for non-precision approaches, there are (were) 3 precision instrument approach options available:  Precision Approach Radar (PAR), Instrument Landing System (ILS, or “Bullseye”) Automatic Carrier Landing System (ACLS).
PAR: This consists of both azimuth and glide slope radar that allows the controller to issue verbal advisories to “talk down” the pilot.  This is generally a back up for when the other systems might not be working.
ILS: Although it works on different frequencies than civilian ILS, functionally it is identical.  A passive signal is received by the approaching aircraft which powers azimuth and glide slope indicators in the cockpit.
ACLS: This is an active system that locks onto the aircraft and sends a discrete signal to drive the indicators.  For a pilot flying a Mode 2 approach the indications are identical to the ILS, but there is greater precision as well as the option to upgrade to a Mode 1.  In a Mode 1 ACLS approach the pilot can couple the autopilot to the signal to allow it to fly the approach hands-off.
All these options are similar to what civil airfields have available, with one notable exception:  The aircraft carrier has Landing Signal Officers (LSOs) on station.  These hardy souls man the flight deck during all recovery operations, in all kinds of weather, and are the final piece in helping a pilot get aboard when the weather is bad.  In very poor visibility LSOs can see the landing light before the pilot can see the carrier and can help talk them down on short final. (There are lots videos on YouTube of this...)
Because of the unique functionality of the Pilot/LSO team, the concept of “minimums” and “missed approach” don’t really apply at the ship.  At 3/4 mile from touchdown the pilot will be asked by the controller to “call the ball” in reference to seeing the glide slope indicator of the ship’s Optical Landing System.  The pilot will either call the ball in sight, or declare “Clara” which means he/she does not have the ball (ship) in sight.  On the call of Clara the LSO will take over issuing power and line up calls over the radio until touchdown, or instruct the pilot to wave off if a safe landing is not possible.
And finally, if the airplane is bingo fuel they would be expected to divert unless the ship is operating blue water, in which case they would go to the tanker.

Wait, are you saying there's always a tanker circling overhead? Or is that just when the weather's bad?

@TonyK, Yes, every recovery.  But it isn't a jumbo USAF tanker, it is one from the ship.  This was a KA-6 Intruder back in the day, an S-3 Viking with buddy store in my time, and I guess F/A-18s are tanking each other now.  If weather was bad they might launch the alert tanker to have extra gas airborne.

P.S.  The duty tanker pilots were trained so that if they were told by the boss to "hawk" a particular plane, (VFR only) they would be at 1500' up the right side of the ship with their hose out just as it touched down so that if the low-fuel pilot missed catching a wire they could simply raise their landing gear, join up, and start taking gas in under a minute.  This was practiced regularly during predeployment workups and was a sight to behold.

Fascinating stuff! What was your role?

EA-6B Prowler pilot and LSO.

The tanker itself has to land somehow / eventually, right?  Worst case, does it just loiter until the weather is slightly less bad, or does it keep making attempts while it has plenty of fuel?  Or something else?  It depends on the situation and weather forecast, I'd assume.

Yes, the recovery tanker always lands last.  If the planes ahead of it can land then it can too.

How is this done when without giving away the location of the aircraft carrier with radio signals?

@IanRingrose Aircraft carriers aren't exactly stealth vehicles - they're huge, and usually escorted by a large fleet of other ships for protection.  There also aren't that many of them in the world (less than 50 including every navy in the world).  It's generally expected that every country with a vested interest probably knows roughly where all of them are at any given time.

There is an EMCON (emissions controlled) recovery procedure, but the weather needs to be good enough to see the ship.

Although I can’t detail fixed wing operations at sea, many countries operating helicopters use an ELVA procedure, an Emergency Low Visibility Approach. Most vessels operating aircraft will have a radar to provide a SCA (ship controlled approach) or if the helicopter has a radar it will be able to fly its own HCA (helicopter controlled approach). Assuming the helicopter can’t get visual using these facilities (with
approach and deck lighting turned up full etc) an ELVA would be used. This involves the helicopter approaching the vessel from 180-deg at approx 2nm astern at deck height plus 10-20ft. For a carrier this would be about 60-70ft. The helicopter would normally be using a radalt height hold at this point and would close the ship at a slow speed. The ship would be dropping marker flares into the water from the stern, as the aircraft gets closer the closing speed would be reduced to 10-15kts above the ship speed until visual. Unlike a radar controlled approach, there is no planned go-around for this procedure however, if a land based diversion was available, this would be used first.


text:
What is the meaning of “next frequency” in PDC？
When we get pdc we see next frequency,For example  121.4
Sometimes it is the frequency of clearance,but sometimes is the frequency of departure.
WHY?Which frequency should it be?

Hi Afanti, welcome to aviation.stackexchange.com! Can you explain what  PDC is? Perhaps give an example (screenshot, picture, verbatim text) to provide context to your question. It will increase the chance of getting a good answer.

And are you seeing this in actual PDCs, or in a flight simulator PC game?  It makes a difference...

I use PDC some (CPDLC more often), and that "next frequency" line isn't something I recall seeing.  Please provide a screenshot of an example, and tell us where you're getting this (i.e. what country/airport).  This may be something used locally in certain places; in the US, I've not seen it.

In east asia airport,For example VHHH RJTT,We request PDC(Pre-depart clearance) via DCL,We get the message like:"Clear to destination Via....NEXT FRQ 121.4",Sometimes the Frequency  is delivery but sometimes the frequency is departure,Which frequency should it be? Is there any regulation about it?

Are you getting the "next freq" message at many airports, in multiple countries? Possibilities would include, Country (or airport) A uses that for one thing, while B uses it for another, or it shows Clearance Delivery in cases where that's your next required call, but Departure otherwise. Does it ever give a Ramp or Ground Control freq? Until somebody else who flies in that region can chime in, best advice is to see if you can spot a pattern to it.

I do know how to send a picture but I have a pdc printed:
^^^^^^^^^^^^^^^^^^^^^^^
FROM LFPG
DEPARTURE CLEARANCE
CLD 1117 220222 LFPG PDC 040
XXX7224 CLRD TO TJAA OFF 27L VIA RANUS5A SQUAWK 5062 ADT 11:30 NEXT FREQ 121.810 TSAT 11:30 CONFIRM FULLY READY ON 121.730
(RECEIVED AT 1117Z)
STS:OPEN
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

the 121.810 here is GROND FREQ


text:
what is ''extended runway centerline''?
When we calculate performance with fly smart，we see：‘‘Take off based on climbing on extended runway centerline‘‘.
What is the meaning of‘‘extended runway centerline‘‘？How long is Extended？

What is "fly smart"?

The centerline of the runway is exactly what it sounds like: an imaginary line running right down the center of the runway. The extended runway centerline is what you get if you extend that line beyond the end of the runway. If you're on the extended centerline, and you look at the runway, you'll be looking at it directly end-on.
The extended centerline doesn't end. Since it's imaginary, it can keep going to infinity in both directions. But, it becomes less and less important to navigation as you get further from the runway (and, if you're not taking off or landing on that runway, it's not really important for anything at all).

Is the centerline itself imaginary? I thought it was painted on the runway.

@Someone Not all runways have centerline markings, especially soft-field ones. But the line itself always exists, at least as an idealized mathematical construct

The extended runway centerline can be very important for ATC operations, and it is not unusual for ATC to issue instructions (to helicopters, e.g.) like "remain north of the Runway 16 extended centerline" or "proceed directly across the Runway 35 centerline, then continue on course."


text:
Airbus pilot Standard call out
For airbus pilots,when they Encounter a Master caution or master warning,what is PF's Standard call out?
"I have a control and communication,ECAM actions.",
OR just "ECAM Actions."?

After having announced the failure it's  ' My Radios, ECAM actions' No need to say 'I have control' if you're the PF!  Other operators SOP's may vary.


text:
Boeing 747 Autopilot PID Controller
For a personal flight simulator project, I am creating my own custom Boeing 747 classic series. I have about 50% of the project complete at a level of detail nearly equivalent to study-level flight simulators. I hope this is the right place to ask this.
Currently I am writing an autopilot that utilizes PID controllers. PID's are new to me so I am still learning a lot. I will do my best to explain what I need. If I am incorrect somewhere, please correct me!
The first controller I am attempting to write is for Heading Select, which will cause the autopilot to bank the airplane such that it will obtain and then maintain a selected magnetic compass heading. I have a lovely NASA paper, formed by collaboration with NASA and Boeing, that gives excruciating detail of the gains used for all autopilot modes. I also am referencing David Allerton's Principles of Flight Simulation for some basic examples.
According to NASA, for all lateral (roll) autopilot modes, the following fixed attitude and rate gains are used:
$\frac{\delta_a}{\phi}=3.2\frac{deg}{deg}$ and $\frac{\delta_a}{\dot \phi}=3.6\frac{deg}{deg/sec}$
This part already has me a small bit confused, but according to a later page, these are the gain values for roll angle and roll rate, respectively. The autopilot is limited to 12 degrees of aileron and 26 degrees of aileron deflection per second.
Source: https://ntrs.nasa.gov/api/citations/19730001300/downloads/19730001300.pdf
Pages 543 and 553
My best guess as to what these were, were the P and D values for an aileron controller (P value of 3.2 * bank error, D value of 3.6 * roll rate) but I am not sure of this. Testing the PID controller with this method, the aircraft does indeed roll to assigned headings, but the roll rate can be very high and does not seem well controlled.
The whole controller looks like this. The gain value of 2.83 is directly from page 553 listed above for the gain value of Heading Select mode on bank angle calculation.
$0.0016175\mathit{V}+0.110375 $ minimum $0.353$ maximum $1.0$ - This is a multiplier on the gain for heading select mode, proportional to true air speed (V)
$2.83$ multiplied by that True Air Speed modifier is the gain for Heading select, $K_p$
$K_p$ * heading error = target bank angle, $\phi$, maximums $\pm 30$ degrees left or right
Target Bank angle - Current Bank = Bank error
Bank error * $0.1$ = Target Roll Rate, $\dot \phi$, maximums $3$ $deg/sec$
Target Roll Rate - Current Roll Rate = Roll Rate Error
Bank error * $3.2$ = Proportional aileron deflection $P$
Roll Rate * $3.6$ = Derivative aileron correction $D$
$P+D$ = Total aileron deflection, maximums $\pm 12$ degrees
So that leaves me with figuring out how to get the deflection rate (how quickly the autopilot moves the ailerons). I am not sure how to calculate that with what I have and I find that my autopilot is completely ignoring the maximum roll rate information, accelerating very quickly into a bank.

Paging @Terry to the white courtesy phone. Terry to the white courtesy phone for a 747 question, please. Sadly, that probably won't work as I haven't seen him around in quite a while, but he'd be your guy - he spent about a decade as a 747 pilot/co-pilot and wrote a fair bit of software about it and for other aircraft. I'm sure someone else will give you a good answer too.

@FreeMan indeed Terry hasn't been around for a while. His own website says he hasn't updated his 'I-am-still-alive-date' in 10 days. I hope he's doing ok.

That's a shame... Any idea where I could potentially reach Terry these days?

@ChristopherHelton I think you are missing the rate control loop. Normally you would first have an roll angle PID, which determines the amount of roll rate command (which you can additionally limit, resulting in a slower response in roll) and then another PID which determines the amount of aileron deflection in order to follow the aforementioned roll rate command